1. Field of the Invention
Communications and data transmission systems which transmit information signals in the form of optical pulses over a dielectric waveguide such as an optical fibre are now commonplace. Whilst improvements in the sources of the optical pulses and in the optical fibre waveguides have increased the range over which such signals can be transmitted to between 100 and 200 kilometers it is still necessary to regenerate the signals when they are transmitted over greater distances and when their power is reduced by beam splitting, being switched or otherwise handled.
In this specification, the term optical is intended to refer to the visible region of the electromagnetic spectrum together with those parts of the infrared and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
2. Related Art
The applicant's patent U.S. Pat. No. 4,879,761 granted on Nov. 7, 1989 describes a regenerator in which the regeneration takes place in the optical domain. This obviates the need for optical to electrical conversion by a photodiode which electrical signal is then amplified and reshaped in an electronic regenerator circuit before being converted by an optical source into an optical pulse once again for onward transmission along the next optical fibre transmission line.
The above optical regenerator comprises a resonant laser amplifier biased to a level just below the lasing threshold of the laser amplifier. Clock signals are coupled to the amplifier having a power just below the optical bistable threshold of the amplifier so that when an optical information signal is also coupled to the amplifier during the application of a clock signal, the bistable threshold of the amplifier is exceeded causing a sudden jump in the power of the optical output of the amplifier to provide a regenerated optical information signal.
It should be noted that the lasing threshold and the bistable threshold apply to distinct phenomena. The electrical bias applied to the laser amplifier is below the lasing threshold bias current and consequently the amplifier does not lase. The sudden amplification occurs because the optical power of an input optical signal is made to exceed an optical power bistable threshold for the amplifier.
When the optical power input to a semiconductor laser amplifier is increased, the extra stimulated emission raises the recombination rate and the carrier density is correspondingly reduced. As a result, the effective refractive index of the active region of a resonant laser amplifier increases with the optical power passing through it. The amplifier resonances are thereby tuned to longer wavelengths, and the gain at a given wavelength there fore varies. The power transfer characteristics of such an amplifier are consequently non-linear and, at appropriate input wavelengths, bistable operation is made possible.
Regeneration using the above described regenerator is achieved by combining an information signal that is to be regenerated with an optical clock signal and coupling them both into the resonant, non-linear optical amplifier.
The clock signal consists of a train of optical pulses at the desired regeneration rate and with a wavelength at which the amplifier is bistable. The peak power of the clock signal is held marginally below the bistable threshold optical power level at which the amplifier will jump into a higher gain state.
With a low power information signal the output of the amplifier is in a lower gain state and its output comprises the slightly amplified clock signal. When the information signal increases to a high enough level such that the power in the combined information and clock signals is sufficient to exceed the bistable threshold, the resonant wavelength of the laser amplifier is suddenly matched to that of the light passing through it and the amplifier jumps into the higher gain state. It remains in this state, even if the input signal level again falls, until the end of the current clock signal. The output of the amplifier for that period then includes a highly amplified clock signal.
The complete regenerated signal comprises a train of return-to-zero pulses with the timing and wavelength of the clock signal.
In this prior art regenerator of U.S. Pat. No. 4,879,761 the information signal need not be at the same wavelength as the clock signal as long as it is shifted from the output wavelength by a multiple of the amplifier mode spacing to ensure that the amplifier has adequate gain at the information signal wavelength to trigger the bistable operation.
The wavelength of the clock signals and their power must be closely controlled to lie close to the bistable threshold. This wavelength is dependent on the amplifier resonant mode and control can be maintained relatively easily due to the colocation of the source of the optical clock signal, and the regenerator. The wavelength of the information signal is not so critical--some variation is possible while still obtaining sufficient gain from the resonant amplifier to trigger the bistability--but the control is more difficult due to the remoteness of the source of the information signals from the regenerator.